How to Make Great Data Visualisations

Aurélien Goutsmedt
aurelien.goutsmedt@uclouvain.be

ICHEC Brussels Management School

UC Louvain, ISPOLE

Thomas Laloux
thomas.laloux@uclouvain.be

UC Louvain, ISPOLE

April 30, 2025

What makes a great data visualisation?

One thing’s for sure: it’s never a pie chart…

General Considerations

Healy (2018) Data Visualization: A Practical Introduction. Princeton University Press.

  • General discussion on commong issues regarding data visualizations
  • Applications to social sciences
  • Focus on R package ggplot2 (Wickham, 2016)

Why visualizing data?

  • Exploring your data (identifying patterns and problems, etc…)
    • Extract information intuitively, efficiently, and accurately

  • Discovering relationships in your data
  • Communicating information with precision, in a concise way
  • Displaying your main results
  • More sociological dimensions: bolster credibility
  • … but, it is not sufficient to “look at data” for good analysis, and anyway, it comes with some risks

Edward Tufte’s main principle

Graphical excellence is the well-designed presentation of interesting data—a matter of substance, of statistics, and of design. … [It] consists of complex ideas communicated with clarity, precision, and efficiency. … [It] is that which gives to the viewer the greatest number of ideas in the shortest time with the least ink in the smallest space. … [It] is nearly always multivariate. … And graphical excellence requires telling the truth about the data. (Tufte, 1983, p. 53)

Other criteria

Three hurdles towards great data visualisation

Bad tastes

Bad data (use)

Perception issues: colors

Perception issues: shapes and scales

#Part 2: Practical Data Visualization with ggplot2

What is ggplot2 ?

  • ggplot2 is a data visualization package for R developed by Hadley Wickham.
  • It lets you build complex plots from simple building blocks.
  • Inspired by the Grammar of Graphics (Wilkinson, 2005) — a system for describing and thinking about graphics.

The grammar describes a consistent syntax for the construction of a wide range of complex graphics by breaking them into simple parts, such as data, shapes, colors, and labels. By enabling the concise specification of these components, it shifts the focus from choosing predefined chart types to systematically defining what to show and how to show it

Why should you use ggplot2?

  • Powerful and flexible
  • Grammar of Graphics: Customize almost every visual aspect
  • Intuitive syntax once you understand the grammar
  • Active community, extensive documentation, updates, and extension (packages)
  • Integrated with R and the tidyverse suite

ADD EXAMPLES OF FIGURES (NICE ONES)

How ggplot2 Works

Plots as a combination of different layers

  • ggplot2 builds plots by layers, each layer adds a different component to the plot
  • The concept behind ggplot2 divides plot into three different fundamental parts: Plot = data + Aesthetics + Geometry.
  • Then you customize plot details (the theme) with elements e.g., the scales, grid, labels, and legends using specific functions

=>Plots are built incrementally by adding components piece by piece

How ggplot2 Works

Three Main Components of ggplot2

  • Data: The dataset used to generate the plot.
  • Mapping: Specifies which variables from the dataset are mapped to visual properties (aesthetics) like position, color, or size.
  • Geom: Refers to the geometric objects (e.g., points, lines, bars) that represent data on the plot=>The type of plot you want to create. You can combine multiple geoms: e.g., points with a regression line.
  • Then the details can be improved!

Examples

Presenting the dataset

Publications by ISPOLE researchers, downloaded from DIAL

 glimpse(data_ispole)
Rows: 2,779
Columns: 50
$ id                                 <chr> "boreal:281323", "boreal:299186", "…
$ type_de_publication                <chr> "Article de périodique (Journal art…
$ sous_type_de_publication           <chr> "Article de recherche", "Article de…
$ sous_type_specifique               <chr> "", "", "", "", "", "", "", "", "",…
$ annee                              <chr> "2025", "2025", "2025", "2025", "20…
$ statut_de_publication              <chr> "Accepté/Sous presse", "Publié", "A…
$ type_d_acces                       <chr> "Accès interdit", "Accès libre", "A…
$ entite_s_departement_s             <chr> "UCL - SSH/SPLE - Institut de scien…
$ auteur_s                           <chr> "Dehoux, Amaury", "Bocquet, Nicolas…
$ editeur_s_directeur_s_scientifique <chr> "", "", "", "", "", "", "", "", "",…
$ collaborateur_s                    <chr> "", "", "", "", "", "", "", "", "",…
$ traducteur_s                       <chr> "", "", "", "", "", "", "", "", "",…
$ prefacier_s                        <chr> "", "", "", "", "", "", "", "", "",…
$ promoteur_s                        <chr> "", "", "", "", "", "", "", "", "",…
$ titre                              <chr> "\"Comme nous existons\" de Kaoutar…
$ language                           <chr> "Français", "Anglais", "Français", …
$ editeur_commercial                 <chr> "", "Regulation & Governance", "", …
$ lieu_d_edition                     <chr> "", "Australia", "", "Den Haag", ""…
$ pagination                         <chr> "Vol. 79, no. 2, p. à paraître (202…
$ doi                                <chr> "", "https://doi.org/10.1111/rego.1…
$ url                                <chr> "http://hdl.handle.net/2078.1/28132…
$ url_pubmed                         <chr> "", "", "", "", "", "", "", "", "",…
$ titre_du_periodique                <chr> "French Studies", "Regulation & Gov…
$ titre_abrege                       <chr> "", "", "", "", "", "", "", "", "",…
$ issn                               <chr> "0016-1128", "1748-5991", "1777-580…
$ eissn                              <chr> "1468-2931", "", "", "2334-3745", "…
$ ponderation                        <chr> "", "", "", "", "", "", "", "", "",…
$ peer_review                        <chr> "Peer", "Peer", "Peer", "Peer", "Pe…
$ mention_d_edition                  <chr> "", "", "", "", "", "", "", "", "",…
$ collection                         <chr> "", "", "", "", "", "", "", "", "",…
$ isbn                               <chr> "", "", "", "", "", "", "", "", "",…
$ auteur_s_de_l_ouvrage_hote         <chr> "", "", "", "", "", "", "", "", "",…
$ titre_de_l_ouvrage_hote            <chr> "", "", "", "", "", "", "", "", "",…
$ nom_lieu_date_de_la_conference     <chr> "", "", "", "", "", "", "", "", "",…
$ jury                               <chr> "", "", "", "", "", "", "", "", "",…
$ date_de_defense                    <chr> "", "", "", "", "", "", "", "", "",…
$ pays_organisme_brevets             <chr> "", "", "", "", "", "", "", "", "",…
$ numero_brevets                     <chr> "", "", "", "", "", "", "", "", "",…
$ organisme_et_collection_rapport    <chr> "", "", "", "", "", "", "", "", "",…
$ mot_s_cle_s                        <chr> "Harchi ¦ Ernaux ¦ Postmigration ¦ …
$ mesh                               <chr> "", "", "", "", "", "", "", "", "",…
$ jel                                <chr> "", "", "", "", "", "", "", "", "",…
$ c_re_f                             <chr> "", "", "", "", "", "", "", "", "",…
$ lc                                 <chr> "", "", "", "", "", "", "", "", "",…
$ identifiants_institut_pole         <chr> "GLOBALIT", "", "", "", "", "", "",…
$ financement_institution            <chr> "", "", "", "", "", "", "", "", "",…
$ financement_subvention             <chr> "", "", "", "", "", "", "", "", "",…
$ financement_programme              <chr> "", "", "", "", "", "", "", "", "",…
$ financement_projet                 <chr> "", "", "", "", "", "", "", "", "",…
$ n_titre                            <int> 83, 129, 187, 85, 95, 98, 77, 95, 1…

Example Plot: Number of Publications by Year

Start with the data and the mapping:

  ggplot(data=data_ispole,aes(annee))

Example Plot: Number of Publications by Year

Identify the Issue : Bad Encoding?

annee
Delreux, Tom;";";";";";One big conversation: the EU’s climate diplomacy across the international regime complex on climate change: the case of the Paris Agreement Negotiations.;Anglais;";";";";http://hdl.handle.net/2078.1/260592;";";";";";";";";";";";";The European Union in International Affairs, Online, du 26/05/2021 au 28/05/2021;";";";";";European Union ¦ climate diplomacy ¦ international climate politics;";";";";CEE;";";";" boreal:270632;Communication à un colloque (Conference Paper);Présentation orale avec comité de sélection;";2021;";";UCL - SSH/SPLE - Institut de sciences politiques Louvain-Europe;Squevin, Pierre Louis Daniel
Cambré, Bart
Rihoux, Benoît;";";";";";The impact of the Stemtest/Test électoral on voting in the Belgian general elections of 2019;Anglais;";";";";http://hdl.handle.net/2078.1/224165;";";";";";";";";";";";";the Eighth edition of the conference Belgium: The state of the federation, Bruxelles, 19/12/2019;";";";";";";";";";";CESPOL;";";";" boreal:224208;Communication à un colloque (Conference Paper);Présentation orale avec comité de sélection;";2019;";";UCL - SSH/SPLE - Institut de sciences politiques Louvain-Europe;Moyson, Stéphane;";";";";";The individual psychology of policy learning in the advocacy coalition framework: Information use, egocentrism and self-esteem in the European liberalization of Belgian network industries;Anglais;";";";";http://hdl.handle.net/2078.1/224208;";";";";";";";";";";";";General Conference of the European Consortium for Political Research (ECPR), Wroclaw, Poland, du 04/09/2019 au 07/09/2019;";";";";";CMAP/POL;";";";";SPLE;F.R.S.-FNRS;ECODEMO;PDR;ECODEMO boreal:222588;Communication à un colloque (Conference Paper);Présentation orale avec comité de sélection;";2019;";";UCL - SSH/SPLE - Institut de sciences politiques Louvain-Europe;Ait-Chaalal, Amine;";";";";";The interactions between globalization, inequality and (in)security in the European, African and Middle Eastern contexts: traditional views and new problems. ;Anglais;";";";";http://hdl.handle.net/2078.1/222588;";";";";";";";";";";";";New challenges for the 21st Century, Universidade de Brasilia , 02/10/2019;";";";";";Europe Middle East Africa International Relations;";";";";ISPOLE ¦ CECRI;";";";" boreal:243889;Communication à un colloque (Conference Paper);Présentation orale avec comité de sélection;";2019;";";UCL - SSH/SPLE - Institut de sciences politiques Louvain-Europe;André, Lyla;";";";";";The role of the EU as a leading donor in framing the humanitarian response to the Syrian “Refugee crisis” in Lebanon;Anglais;";";";";http://hdl.handle.net/2078.1/243889;";";";";";";";";";";";";EISA Annual Conference A Century of Show and Tell: The Seen and the Unseen of IR, Sofia, Bulgaria, du 10/09/2019 au 14/09/2019;";";";";";European Union - Humanitarian aid - Refugee protection;";";";";European Management Syrian refugee crisis;Université Catholique de Louvain;FSR;ISPOLE / Germac;"The Eu management of the Syrian refugee crisis" boreal:215145;Communication à un colloque (Conference Paper);Présentation orale avec comité de sélection;";2019;";Accès libre;UCL - SSH/SPLE - Institut de sciences politiques Louvain-Europe;Reuchamps,
Van Ingelgom, Virginie
mémoire
Burns, Charlotte;";";";";";The involvement of the European Parliament in UN climate negotiations;Anglais;";";";";http://hdl.handle.net/2078.1/200022;";";";";";";";";";";";";European Union in International Affairs VI, Brussels, du 16/05/2018 au 18/05/2018;";";";";";Climate change ¦ Climate Diplomacy ¦ Conference of the Parties (COP) ¦ European Parliament ¦ European Union ¦ parliamentarization ¦ United Nations Framework Convention on Climate Change (UNFCCC) ¦ climate change ¦ climate diplomacy ¦ Conference of the Parties ¦ European Parliament ¦ European Union ¦ parliamentarization ¦ Unitied Nations Framework Convention on Climate Change;";";";";CEE;";";";" boreal:200030;Communication à un colloque (Conference Paper);Présentation orale avec comité de sélection;";2018;";Accès restreint;UCL - SSH/SPLE - Institut de sciences politiques Louvain-Europe;Delreux, Tom

Example Plot: Number of Publications by Year

(re)Start with the data and the mapping:

data_ispole%>%
  mutate(annee=as.numeric(annee))%>%
  ggplot(aes(annee))

Example Plot: Number of Publications by Year

Now add the geom

data_ispole%>%
  mutate(annee=as.numeric(annee))%>%
  ggplot(aes(annee))+
  geom_bar()

Example Plot: Number of Publications by Year

Now add the geom (bis)

data_ispole%>%
  mutate(annee=as.numeric(annee))%>%
  ggplot(aes(annee,fill=peer_review))+
  geom_bar()

Example Plot: Number of Publications by Year

Now add the details

library(openxlsx)
library(ggpubr)
library(hrbrthemes)
library(ggsci)
library(see)
library(gghighlight)
  
data_ispole=data_ispole%>%mutate(annee=as.numeric(annee))

Example Plot: Number of Publications by Year

Now add the details

data_ispole%>%
  ggplot(aes(annee))+
  geom_bar(fill='darkseagreen',color="black")+scale_x_continuous(breaks=c(2010,2024))+
  theme(
    panel.grid.major.y=element_line(color="black",linetype="dashed"),
    panel.grid.minor.y=element_line(color="red",linetype="dotted"))

Example Plot: Number of Publications by Year

Now add the details

data_ispole%>%
  ggplot(aes(annee))+
  geom_bar(fill='darkseagreen',color="black")+
  theme_pubclean()+
  theme(plot.title =element_text(size=20,hjust=0.5))+  labs(x=NULL,y=NULL,title="Evolution of ISPOLE's Publication Activity Over Time" )

Title Length Across Publication Types

  • What would be a good way to visualize this?
data_ispole%>%select(type_de_publication,n_titre)%>%glimpse
Rows: 2,779
Columns: 2
$ type_de_publication <chr> "Article de périodique (Journal article)", "Articl…
$ n_titre             <int> 83, 129, 187, 85, 95, 98, 77, 95, 127, 77, 62, 98,…

Title Length Across Publication Types

  • What would be a good way to visualize this?
data_ispole%>%
  ggplot(aes(x=type_de_publication,y=n_titre))

Visualizing Title Length Across Publication Types

  • Visualize the data through a boxplot
data_ispole%>%
  filter(
     type_de_publication=="Monographie (Book)"|
       type_de_publication== "Contribution à ouvrage collectif (Book Chapter)"|
       type_de_publication== "Article de périodique (Journal article)"|
     type_de_publication== "Thèse (Dissertation)"|
       type_de_publication== "Communication à un colloque (Conference Paper)"|
     type_de_publication== "Document de travail (Working Paper)"
   )%>%
  ggplot(aes(x=type_de_publication,y=n_titre))+
  geom_boxplot()+
theme_pubclean()#
  #coord_flip()

Visualizing Title Length Across Publication Types

  • How could we improve this plot?

Visualizing Title Length Across Publication Types

  • Spot the modifications!

Visualizing Title Length Across Publication Types

  • Add Faceting by Another Variable (and Another Geom)

Visualizing Title Length Across Publication Types

  • Add Faceting by Another Variable (and Another Geom)
p+
  geom_jitter(show.legend = F,alpha=0.5,shape=21,size=0.2)+
  facet_wrap(.~peer_review)+
  theme_ipsum(base_family = "Century Gothic")+
 theme(axis.title.y = element_blank(),
         plot.title.position = "plot",  # Align relative to full plot area
        plot.title = element_text(hjust = 0))

General Rule

  • (Almost) Always Use Faceting When Showing Differences Across a Variable

General Rule

  • (Almost) Always Use Faceting When Showing Differences Across a Variable

Exercice

On Paper (No Computers)

  • How would you visualize the relationship between the number of conference papers and the number of published outputs (books, journal articles, and book chapters)?

  • How would you compare peer-reviewed and non–peer-reviewed publications in this context?

  • In both cases, how would you structure your ggplot2 code accordingly?

Exercice

Exercice

References

Healy, K. (2018). Data visualization: A practical introduction. Princeton University Press.
Tufte, E. R. (1983). The Visual Display of Quantitative Information (1st edition). Graphics Press USA.
Wickham, H. (2016). ggplot2: Elegant graphics for data analysis. Springer-Verlag New York. https://ggplot2.tidyverse.org